Method and bath for chemically plating copper



United States Patent Oil 3,3 7 7,1 74 Patented Apr. 9, 1968 ice ABSTRACTOF THE DISCLOSURE A process and'bath for chemically. plating copper on anon-electric-conducting substrate in which chelating agents "such as.2,2'-'hiquinoline, 2,9-dimethylphenanthroline and'4,7-diphenyl-2,9-dimethyl+l,10-phenanthroline are'used in the bath. Thechelating agent forms a tetrahedral chelated compound selectively withcuprous ion which thereby allowsfor a control ofthe cuprous ion contentof the bath in the vicinity of the substrate being plated, which in turnleads to a control of the velocity of a reduction reaction-betweenformaldehyde and certain cupric ion complex compounds which latterreaction provides the copper-used in the plating process.

'The present invention relates to a method for chemically plating copperand the bathused therefor.

In executing various metal platings on such non-electric-conductivematerials as synthetic resins, glass, porcelains, wood, etc., amethod'is used in which generally, the non-electric conductive materialsare first chemically plated with copper, then the resultant copper layeris thickened by electrical copper plating, and then on this surface,arbitrary metals are plated.

This chemical copper plating method has been long known as a method forplating non-electric-conductive .mateiialsf'lhe well-known copperplating bath consists of .an alkaline aqueous solution containingWater-soluble copper salts, complexing agents for forming complex com-.poun-dsof cupric ion and formaldehyde which is used as a reducing agentfor precipitating metallic copper by reducing the above-mentionedcomplex compoundsQIn order to make the above-mentioned reducing reactionwith formaldehyde proceed, a catalyser of a sort is required and thesurface to be plated is preliminarily subjected to an activatingorsensitizing treatment. When the abovementioned treated material to beplated is immersed in the bath, metalliccopperdeposits'on-thevmaterialto be plated by the reducing action of the formaldehyde, and thereafter,this deposited metallic copper provides the -catalysing'actionby itself,whereby the platingis carried on I to completion.

However, various difficulties are encountered in :platling bythismethod. The first difiiculty lies in the fact that :the decompositionvelocity of the solution is very high, the solution decomposing onlylllaflfW hours, resulting in the impossibility of plating, and moreover,this high reducing reaction velocity cannot .be controlled. The seconddifficulty. liesinthefactlhat with the lapse of time, "the surface.ofthe .copper depositing on the material to be plated becomesroughened, and .the surface cannot :be maintained ina lustrouscondition, with the result that in the subsequent electricplatingprocess, a lustrous plated surface is difficult'to obtain, andmoreoven the coherency of the coatings becomes inadequate. The thirddifliculty lies in the fact that cuprous oxide is formed as a'byproduct,which mixes into the plated copper surface, embrittling the surface tobe plated.

Accordingly, an object of'this invention is to provide a new, improvedchemical copper plating method using a bath which permits appropriatecontrol of the reduction reaction velocity, and accordingly, is improvedin its abilityto endure long continuous use.

Another object of this invention is to provide a chemical copper platingmethod and the bath used therefor whereby the plated layer obtained isalways fine in texture and lustrous, thereby allowing one to obtain uponsubsequent electrical plating a plated surface having good coherency andluster.

A further object of this invention is to provide a chemical copperplating method-and the bath used therefor whereby the copper ion in thebath may be consumed quite economically, by only depositing metalliccopper on .the material to be plated, and by avoiding the production ofby-product cuprous oxide.

These objects and other objects of this invention which will becomeapparent in view of the following descriptions may be achieved with achemical copper plating bath which is an alkaline aqueous solutioncontaining water-soluble copper salt, cupric ion CompleXing agentandformaldehyde, to which is added a small amount of .a chelatingreagent .which forms a tetrahedral chelated compound selectivelywithcuprous ion.

The present invention is-based on thediscovery that by the addition ofthe above-mentioned specified chelating reagent to the chemical copperplating bath,.the cuprous ion concentration inthe vicinity of thesurface of the material to be plated can be controlled, and as result,the reduction reaction velocity can be readily controlled, improvingupon the stability of the plating bath, and it is thereby made possibleto use the plating bath continuously for periods of time that are morethan 20 to 40 times long- .erthan the periods of time in which theconventional treating baths have been used and wherein the conventionalbath usually becomes unusable only in a few hours; as well as on thediscovery that more than about 90% of the copper ion .existingin thebath may be consumed without loss, andmoreover, theplated'layer therebyobtained is always time in texture and highly lustrous,-and 'hasgoodcoherency with respect to Contact with the layer subsequentlyproduced by the electrical plating, giving a lustrous finished platedsurface.

Among various reagents which form o'helated compounds with copper ion,those exhibiting such an effect are limited, for thepurposes of thepresent invention, to the chelating reagents capable of forming atetrahedral chelated compound selectively with cuprous ion only. Otherreagents, .for example, dithiocarbamic acids, dithizone and itsderivatives, 'form stable chelated .com- ,pounds with cupric ion ratherthan with cuprous ion, and are therefore unable .to control thecuprousion concentration. Withthem, the object ofthis invention cannotbe achieved.

Representative chelating agents which may be used asilllLlStP-fltlOIlSOf the chelating agents Which-may be used -inathe:present invention include reagents 'c-apableofform- *ing'chelatedcompounds with :cuprousion by interposing .a nitrogen atom; .e.g.,pyridyl derivatives :such .as 2,2 biquinoline, etc., and phenanthrolinederivatives suchas 2,9 dimethylphenanthroline, 4,7-diphenyl2,9-dimethyl- -1,l0-'phen-anthroline,- etc.

The desirable amount of each component in the chemical plating bathsolution'of this invention is "as follows:

co'mpounding'material: Proportion '(mols/l.)

Chelating reagent 0.0000l0.001

In addition to the above-listed materials, alkali metal hydroxides andsalts, such as sodium hydroxide and sodium carbonate, etc., are requiredto be employed to make the pH of bath alkaline.

The alkali metal hydroxides and salts should be used in such amounts asto make it possible to adjust the pH of the bath to 10.5-14, and todesirably 11.0-13.0, when the above-listed compounding materials arediluted with water to make a 1 liter solution. The desirable amount ofreagent used to form the chelated compound selectively with cuprous ionin this invention is 0.00003-0.0001 mols./litcr. In the above-mentionedcompounding ratio, the complexing agent is always required to be used inamounts corresponding to 2-5 times in mols the amount of the copper saltemployed.

As the water-soluble copper salt, copper sulfate, cupric chloride,cupric nitrate, cupric acetate and cupric gluconate, may be used, theespecially desirable one being copper sulfate. As the complexing agent,there can be made use of various materials, which are also watersolubleand capable of forming complex compounds with cupric ion in an alkalineaqueous solution having a pH in the range of from 10.5 to 14 andrepresentative materials of such complexing agents are Rochelle salts,sodium citrate, sodium gluconate, ethylenediamine tetraacetic acid,triethanolamine, etc. be mentioned. For the formaldehyde, 30-40%formalin is generally employed, and besides it, paraformaldehyde andtrioxane may be used. In whatever form it may be used, the amountthereof used should be so chosen that the concentration of formaldehydein the bath falls within the abovementioned range.

As the material to be plated, non-electric-conductive materials, such asvarious thermoplastic or thermosetting synthetic resins, for example,polyamide, polystyrene, acrylic resins, epoxy resins and phenol resins,etc., and glass, porcelains as Well as wood, etc., are made use of.

In plating these nonelectric-conductive materials, using the platingbath of this invention, the surface of the material to be plated issubjected to the Well-known pretreatment for activation or sensitizationso as to first form on such surfaces films of various metals whichexhibit the catalytic action required for the reduction reaction of thisinvention, such metals including palladium, gold, platinum, silver,rhodium, copper, cobalt, and nickel, etc. Thus the surface of thematerial to be plated is first uniformly roughened by liquid honing orbarrel tumbling. The liquid honing is carried out, for example, byplaying onto the surface of the material to be plated water containingthe honing agent, and in barrel tumbling, a method is employed by whichthe surface of the material to be plated is abraded and roughened by theuse of a grinding material. Then, the surface treated as abovementionedis washed with water desirably containing a neutral detergent.Subsequently, this washed surface is subjected to a chemical etchingtreatment. For this treatment, various methods may be employed, andrepresentatively, a method is employed in which the surface to be platedis treated with a solution having dissolved therein 50-250 g./chromicacid and 500-1000 ml. of concentrated sulfuric acid per liter of water.Then, after the treated surface is rinsed, this washed surface istreated, to be sensitized, with a solution having dissolved therein5-150 g. of stannous chloride and 40-100 ml. of concentratedhydrochloric acid per liter of water. Further the surface to be platedwhich has been subjected to the above-mentioned treatment is, afterrinsing, treated with a solution having dissolved therein 0.045-1.8 g.of palladium chloride per liter of water to form a palladium film on thesurface, followed by sensitizing or activating the palladium film,followed by rinsing. It is to be noted further that palladium chloridemay be substituted by chloroauric acid (HAuCl for example, or othermetal salts that will provide the desired catalytic action.

The above-mentioned pre-treatment illustrates only one example of suchprocedures", beside it, by various other methods such as shown in MetalFinishing, June, 70 (1962), the metal catalyst film may be formed on thesurface of the non-electric-conduetive material to be plated.

The chemical copper plating method of this invention is executed byimmersing the material to be plated which has been subjected to theabove-mentioned activation treatment in the plating bath of thisinvention. At this time, the bath temperature used is 15-40 C.,desirably 20-35 C. This plating process is finished usually in 3-10minutes, and a plated product having fine textured and lustrous surfaceis obtained. In this method, the bath has a useful life of about twoweeks of continuous plating operations during which a succession ofobjects may be plated therein.

In the following example there are given embodiments of the presentinvention. These examples are included to help understand thisinvention, and the applicable scope of this invention is not therebyrestricted. Modes of application embodying the present invention that donot depart from the spirit of this invention, will be apparent to thoseskilled in the art on the basis of the above explanations and exampleswhich follow:

EXAMPLE 1 The plating surface a polyvinylchloride specimen of x 100 x 2mm. in dimension was roughened by jetting water onto such surface whichcontained one tenth by Weight of 400 mesh emery powder under an airpressure of 3-5 kg./cm. followed by rinsing, and thereafter, wasimmersed for 5 minutes in a solution having dissolved therein 50 g. ofchromic acid and 250 ml. of 98% sulfuric acid per 500 ml. of water, thenwashed with running water. Subsequently, the above-treated specimen wasdipped for 3 minutes in a solution dissolving therein 50 g. of stannouschloride and 100 ml. of 35% hydrochloric acid per liter of water, andwas then rinsed.

The vinylchloride resin specimen subjected to this pretreatment wasplated by immersing it in a plating bath of the following compositionheld at 30 C.:

Water required to make 1 liter solution.

Five minutes thereafter, a plated product of 0.5 1. in thickness withlustrous, fine textured plated surface was obtained. This plating bathwas quite stable, and it was only after two weeks use that smalldeposits of metallic copper were observed on the inner surface of thevessel.

EXAMPLE 2 A 100 x 100 x 2 mm. plate specimen of Kralastic(acrylonitril-butadiene-styrene copolymer resin produced by SumitomoKagaku Kogyo Kabushiki Kaisha, Osaka, Japan) which has been subjected toa pretreatment as in Example 1 was dipped in a chemical plating bath ofthe following composition:

Water required to make 1 liter solution.

Five minutes thereafter, a plated product of 0.511. in thickness withlustrous, fine textured plated surface was obtained. This plating bathwas also quite stable; 10 days Mols Water-soluble copper salt 0.02 to0.35 Formaldehyde 0.5 to 1.5 Cupric ion complexing agent 0.04 to 1.75

the mols of said complexing agent used being 2 to 5 times the mols ofsaid water soluble copper salt used,

0.00001 to 0.001 mol of a chelating agent selected from the group ofcompounds consisting of 2,2'-biquinoline, 2,9-dimethyl-phenanthrolineand 4,7-diphenyl- 2,9-dimethyll, l0-phenanthroline,

such quantities of alkaline compound as are needed to provide saidsolution with a pH in the range from 10.5 to 14, and

such quantities of water as are needed to make a one liter solution.

2. An aquous chemical copper plating bath solution as in claim 1 inwhich said chelating agent is 2,2'biquino line.

3. An aqueous chemical copper plating bath solution as in claim 1 inwhich said chelating agent is 2,9-dimethyl phenanthroline.

4. An aqueous chemical copper plating bath solution as in claim 1 inwhich 0.00003 to 0.0001 mol of said chelating agent are used in eachliter of said solution.

5. An aqueous chemical copper plating bath solution as in claim 1 inwhich such quantities of alkaline compound are used as are needed toprovide said solution with a pH in the range of from 11 to 12.

6. An aqueous chemical copper plating bath solution as in claim 1 inwhich said alkaline compound is selected from the group of compoundsconsisting of the alkali metal hydroxides and carbonates.

7. An aqueous chemical copper plating bath solution as in claim 1 inwhich said Water soluble copper salt is selected from the group ofcompounds consisting of copper sulfate, cupric chloride, cupric nitrate,cupric acetate and cupric gluconate.

8. An aqueous chemical copper plating bath solution as in claim 7 inwhich said water-soluble copper salt is copper sulfate.

9. An aqueous chemical copper plating bath solution as in claim 1 inwhich said cupric ion complexing agent is selected from the group ofcompounds consisting of Rochelle salts, sodium citrate, sodiumgl-uconate, ethylene diamine tetraacetic acid and triethanolamine.

It). An aqueous chemical copper plating bath solution as in claim 9 inwhich said cupric ion complexing agent is Rochelle salts.

11. An aqueous chemical copper plating bath solution as in claim 6 inwhich said Water-soluble copper salt is copper sulfate,

said cupric ion complexing agent is Rochelle salts, and

said chelating agent is 2,2-biquinoline.

12. An aqueous chemical copper plating bath solution as in claim 6 inwhich said water-soluble copper salt is copper sulfate,

said cupric ion complexing agent is Rochelle salts, and

said chelating agent is 2,9-dimethylphenanthroline. 13. A chemicalcopper plating process which comprises immersing, in the aqueouschemical copper plating bath solution of claim 1 maintained at 15 to C.,

the surface of a non-electric-conductive material, said surface havingbeen previously provided With a film of a metal selected from the groupof metals consisting of palladium, gold, platinum, silver, rhodium,copper, cobalt and nickel,

for a period of time sufiicient to allow for the plating of copper inthe desired thickness on said surface.

References Cited UNITED STATES PATENTS 6/1963 Zeblisky et al 1061 7/1966Schneble et a1 1061 X

